Motor drive systems with secondary batteries are used in electric vehicles, hybrid vehicles, railways, or other machines. Such motor drive systems include an inverter that converts direct current into alternating current, and apply the alternating current to an AC (Alternating Current) motor, such as a brushless motor, to thereby drive the motor. These motor drive systems normally use a converter for boosting a battery voltage, and apply the boosted voltage to the inverter, thus achieving higher output power with low loss.
In contrast, there is proposed another type of motor drive systems designed to omit converters so as to reduce its size, weight, and cost. A motor drive system of this type is configured such that one end of each of star-connected three-phase windings of an AC motor is connected with a corresponding AC output of a three-phase bridge inverter and the other ends of the three-phase windings are collectively connected with each other at a neutral point in star-configuration. The motor drive system of this type includes a voltage-variable capacitor connected between the neutral point and a positive or negative DC input terminal of the inverter, and a DC power supply connected across the voltage-variable capacitor. An input capacitor is connected between the positive and negative DC input terminals of the inverter.
The motor drive system of this type is operated as a chopper in zero-voltage mode to control the potential at the neutral point.
Specifically, the motor drive system in the zero-voltage mode turns off, for example, all of the low-side switching elements of the inverter with all of the high-side switching elements being in on state. This allows a substantially direct current to flow, via the neutral point, through each of the three-phase windings and a corresponding one of the high-side switching elements based on the battery voltage. This charges electromagnetic energy in each of the three-phase windings.
Next, the motor drive system in the zero-voltage mode turns on all of the low-side switching elements of the inverter and turns off all of the high-side switching elements. This allows the sum of a voltage generated by the electromagnetic energy charged in each of the high-side switching elements and the battery voltage to be applied to the inverter. An example of the motor drive system of this type, which is operated as a chopper, is disclosed in Japanese Patent Application Publication No. H11-178114.
In the motor drive system of this type, the 3n-th harmonic induced voltages (n is an integer equal to or higher than 1) appear at the neutral point. In order to reduce a zero-phase current component based on the 3n-th harmonic induced voltages through the neutral point, Japanese Patent Application Publication No. 2001-204196 discloses a technique that adds, to three-phase command voltages for the three-phase stator windings, voltage components equivalent to the 3n-th harmonic induced voltages for cancelling the 3n-th harmonic induced voltages. This Patent Publication No. 2001-204196 achieves such an object to reduce the zero-phase current component based on the 3n-th harmonic induced voltages through the neutral point.